What I think most likely happened:
The terrarists were planning an attack on the US for a number of years.
The powers that be knew about this plan, and there is plenty of evidence to suggest this. So, they let it happen with no attempt to stop it, and there is A TON of evidence to support this. Whether or not the government threw in a false flag here and there that day (such as building 7, Pentagon, etc) is debatable..but aside the point.
They let 9/11 happen in order to justify an occupation in the middle east, which to this day has us chasing a bunch of shepherds in the mountains and falsely labeling the vast majority of them as "terrorists
"
Now we have oil companies, security contractors, military contractors etc making BANK from our occupation there. I mean JFC we've been there for a decade, these people are laughing all the way to the bank.
Not to mention we're paying our soldiers 15K a year to guard truck drivers for these companies that make 100K + a year. I mean, come on...
If you somehow think this is far fetched, than you haven't looked at the evidence and the facts and have a very closed mind.